fix: type warning on new_context

[itsaphel]

💡 Motivation and Context

When using a strict Python typechecker, Posthog's new_context function raises a type warning:

Type of "new_context" is partially unknown
  Type of "new_context" is "(fresh: bool = False, capture_exceptions: bool = True, client: Unknown | None = None) -> _GeneratorContextManager[None, None, None]" (basedpyright reportUnknownVariableType)

This fixes that by assigning a type to client

💚 How did you test it?

Type fix only, ran typechecker

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix type warning on `new_context`" | Re-trigger Greptile}

[marandaneto]

@itsaphel can you add a changeset with sampo?

[greptile-apps]

_{Reviews (2): Last reviewed commit: "changeset" | Re-trigger Greptile}